Method of treating electrolytic coatings on magnesium and its alloys



Patented July 8, 1941 COATINGS ON MAGNESIUM AND ITS ALLOYS L Robert W.Buzzard, Remington, Md.

No Drawing.

Original application June 29,

1937, Serial No. 150,932. Divided and this application Apr-i126, 1938,Serial No. 204,387

(Granted under the act of March 3, 1883, as amended April 30, 1928; 8700. G. 757) 6 Claims.

The invention disclosed and claimed herein is a division of my copendingapplication, Serial No.

- 150,932 filed June '29, 1937, now Patent No.

2,203,670 of June 11, 1940. p

This invention relates to the treatment of eleccharacteristics of theelectroylytic coating.

tr-olytically formed coatings on magnesium and its I alloys and one ofits objects is to increase the durability and the protectivecharacteristics of the film by sealing the coating.

,The method specifically relates to the treating of electrolyticcoatings on magnesium and its alloys with a solution in the presence ofheat so as to form a seal that not only increases the durability of thearticle coated to wear but also improves its resistance to corrosion.

The article to be treated in this manner, which has been coatedelectrolytically, is washed free of solution, providing the electrolyticbath is basically difl'erent from the solution used in my sealing methodand is then immersed in a hot solution of sealing compounds, preferablyat or above the boiling point of the solution, but below the fusionpoint of the metal.

The sealing solution should contain a corrosion inhibiting ion such aseither a chromate or dichromate and a second salt as a reacting ion suchas manganese or chromium or borate, phosphate, fluoride or a combinationof any of the said second salts. The concentration of the salts is"limited only by their solubility, but may be varied to conform with'timeof application and quality of seal. Good seals of'coated metal have beenobtained by subjecting the coatings to treatment monobasic phosphate andthe coating has been I washed free of anodizing solution, its corrosionresistance will be greatly improved if it is immersed in a solution ofdichromate and manganous sulphate at .boiling temperature of water orabove. The sealing solution may be varied to contain a trivalentchromium salt as chromium acetate or sulphate or fluoride. A chromatemay be introduced or substituted in the solution or a phosphate may besubstituted or added when the solution is of such composition not tocause detrimental intersolution reactions.

It should be understood that the protection obtained by my treatment isdependent on the presence of an electrolytic coating and is not a primesurface treatment.

In the sealing treatment herein set forth a chemical reaction isproduced improving the This change is evinced by a corrosion testwherein two identical anodized articles, one sealed by my method and oneunsealed, disclosed over a corrosion period that the sealed coating didnot change in color, whereas the unsealed coating showed a radicalchange in color.

As further examples of compositions which will satisfactorily sealelectrolytic coatings 0 magnesium, the following are cited: (a) 1 to 30percent dichromate 0.25 to 10 percent chromium fluoride 10 percentsodium dichromate 1 percent chromium fluoride 10 percent sodiumdichromate 10 percent monobasic sodium phosphate 1 percent chromiumfluoride 10 percent sodium dichromate 1 percent zinc phosphate 1 percentmagnesium phosphate 1 percent chromium fluoride 10 percent sodiumdichromate 10 percent sodium phosphate 1 percent chromium sulphate 1percent'zinc chromate 1 percent manganous sulphate 1 percent chromiumfluoride 10 percent dichromate I 5 percent sodium fluoride .1 percentchromium fluoride 10 percent sodium phosphate 10 percent dichromate 1percent chromium fluoride 1 percent zinc chromate 5 percent sodiumdichromate 1 percent chromium fluoride A double dip may also be usedsuch asfirst immersing the electrolytically coated article in a solutionof dichromate and phosphates 'followed by an immersion in a solution ofmanganese and chromium sulphate. Ammonium and other salts may besubstituted for the sodium and potassiumsalts.

The above mentioned substances used in the magnesium treatments arementioned by way of illustration and not by limitation, since theinvention comprehends treatments of the character indicated broadlyabove and is not to be limited except by the appended claims.

The invention described herein may be manufactured or used by or for theGovernment of the United'States of America for governmental poundselected from group consisting of chromate and dichromate.

2. The method of producing corrosion resistant coatings on' magnesiumand magnesium alloys in. which magnesium predominates comprising formingon the metal an anodic coating produced by reaction with magnesium andsubjecting said coating to a non-electrolytic treatment .in an aqueoussolution containing chromium acetate 20 and a water soluble compoundselected from group consisting of chromate and dichromate.

3. The method of producing corrosion-resistant magnesium or magnesiumalloys comprising forming an anodic coating produced by reaction withmagnesium on the metal and subjecting the coating to a non-electrolytictreatment in an aqueous solution containing a chromium salt 7 soluble inthe solutionand a water soluble compound selected from the groupconsisting of chromate and dichromate, the pH of said solution being onthe acid side.

4. The method of producing corrosion resistant.

coatings on magnesium and alloys thereof in which magnesium predominatescomprising.

' forming on the metal an anodic coating produced by reaction withmagnesium, and subjecting the coating to a non-electrolytic treatment inan aqueous solution containing chromium sulphate v and a water solublecompound selected from the group consisting of the chromates anddichromates, the pH of the solution being on the acid side.

, 5. The method 01' producing corrosion resistant coatings on magnesiumand alloys thereof in which magnesium predominates comprising forming onth metal an anodic coating produced by reaction with magnesium, andsubjecting the coating to a non-electrolytic treatment for 5 to minutesin an aqueous solution containing from 1 to 30 per cent of a watersoluble dichromate and i'rom 0.25 to 10 per cent of chromium fluoride,and maintaining the temperature of the solution during treatmentsubstantially equivalent to the boiling point of the solution.

6. The method of producing corrosion resistant coatings on magnesium andalloys thereof in which magnesium predominates comprising forming on'the metal an anodic coating produced by reaction with magnesium, andsubjecting the coating to a non-electrolytic treatment {or 5 to 30minutes in 'an'aqueous solution containing from 1 to 3 per cent of zincchromate,

3 to '7 per cent of sodium dichromate, and 1 to 3 per cent of chromiumfluoride-and maintaining the temperatureof the solution during treatmentsubstantially equiifalent' to the boiling point of the solution. 1

4 ROBERT W. BUZZARD.

